by Melissa J Luther

Created on: June 16, 2009

Corals have always been under attack by Mother Nature, but they have also always been able to withstand these threats. Now, however, corals may be becoming more susceptible to natural threats as human activity creates additional stressors that weaken the health of the coral animals.

> Natural Threats to Coral Reefs <

* Storms/Hurricanes

Storms are a regular event in the world's oceans, and the waves they create can easily smash fragile coral formations. In addition, heavy coastal rains can wash excessive amounts of sediment, along with freshwater, into the clear saltwater the corals need for optimum health.

* Earthquakes

Just as earthquakes can cause buildings to collapse on land, they can cause corals to shatter or buckle and crack. Corals already weakened by human activity may be less able to recover from these shocks.

* Predators

Predators exist in all ecosystems, but some are capable of destroying entire populations of prey if left unchecked. The crown-of-thorns starfish is one such predator, which can quickly destroy all the living coral on a reef. Invasions of this voracious creature have become regular occurrences on some reefs.

* Disease

Most coral diseases were unknown until a few decades ago, but now new diseases seem to be cropping up with frightening regularity. Scientists know very little about the cause of most coral diseases, but bacteria, fungi and some algae may be to blame for much of the disease.

> Human Threats to Coral Reefs <

* Overfishing

Overfishing disrupts the natural ecosystem and the balance between the residents of a reef. There are fewer fish, and those that remain are smaller juveniles. Overfishing may be the reason the crown-of-thorns is able cause such destruction. Humans have removed most of its natural predators.

In addition, when fish become scarce, fishermen turn to more destructive techniques to try to increase their catch. Some fishermen, especially in Southeast Asia, use explosives to kill fish within a certain area. Unfortunately, the explosion also damages nearby coral, which may not recover for years. With repeated blasts, it may become impossible for the coral to ever recover.

* Pollution

Agricultural runoff and untreated sewage making its way into the water increases the nutrient content of the water. Increased nitrogen concentrations, especially, cause sudden increases in algal growth, called an algal bloom. Depending on the type of algae involved, this overgrowth may block out sunlight,

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